Getting the calibration you need

June 8th, 2016, Published in Articles: EngineerIT

 

If your organisation is like most technology companies, test equipment is an important part of ensuring the quality and performance of your end products. One of the fundamental values of test equipment is the confidence you gain from the measurements – and the pass/fail decisions – they produce.

The source of that confidence is the specified accuracy of each instrument, and the foundation of that accuracy is calibration. Ultimately, the bedrock beneath that foundation is the service provider you select to calibrate your instruments.

Fig. 1: Flow diagram for the calibration process, including repair (if needed).

Fig. 1: Flow diagram for the calibration process, including repair (if needed).

Fig. 2: Flow diagram of a process that will help ensure you get the right calibration for your needs.

Fig. 2: Flow diagram of a process that will help ensure you get the right calibration for your needs.

A quick look at calibration

Calibration is the process of measuring the actual performance of an instrument-under- test (IUT) using lab instruments that have significantly better performance than the IUT. The performance of every lab instrument must be traceable to International System of Units (SI) through a national metrology institute such as NIST, NPL or BIPM.

Calibration involves more steps than you might imagine, and the overall process is shown in Fig. 1. Performance tests compare the instrument’s actual performance to published (i.e., “data sheeted”) specifications. In the ideal case, the service provider will test the performance corresponding to all data-sheeted specifications, for all installed options, every time. If the instrument passes all tests, the process is concluded: The instrument is within specification and you can use it with confidence.

If the instrument is observed to be out-of-specification, the ideal case is for the service provider to perform adjustments and then re-test the instrument. As shown in the diagram, the process either loops back and starts over or, for an instrument that cannot be adjusted into spec, detours to the “hardware repair” path. Repair begins with a diagnosis of the problem and the ordering of necessary parts. Once the repair has been completed, adjustments are performed and the calibration process loops back to the performance tests. After the instrument passes all performance tests, it is once again ready to make accurate measurements. To minimise turnaround time and manage cost, the service provider should be able to perform adjustments and repairs in-house (i.e., without sending the instrument to another service provider).

Table 1. Evaluation questions focused on accreditation.

Table 1. Evaluation questions focused on accreditation.

Table 2: Evaluation questions focused on calibration capabilities.

Table 2: Evaluation questions focused on calibration capabilities.

Table 3: Evaluation questions focused on documentation of calibration processes and results.

Table 3: Evaluation questions focused on documentation of calibration processes and results.

Identifying needs and defining documentation

The starting point for getting the right calibration is identifying your most important requirements. This has three steps:

  • Identify your key instruments
  • Identify the key specifications for each of those instruments
  • Define the types of records and reports you need to document the calibration status of each important instrument and specification

Let’s take a closer look at each of these.

Step 1: Identify key instruments

Every piece of test equipment was purchased for a reason: it may have been an essential feature, capability, measurement, or specification. In most cases, you can quickly identify the high-impact instruments your organisation depends on. Examples include high-performance RF or microwave instruments such as signal analysers, network analysers, signal generators, sampling or real-time oscilloscopes, and high-precision digital multimeters.

Step 2: Identify key specifications

Once you’ve identified the most important instruments it’s time to pinpoint the key specifications each one must meet to help ensure the success of your enterprise. A good starting point is to compile a list of requirements for each instrument. Examples include amplitude accuracy or displayed average noise level in a signal analyser, dynamic range or dynamic accuracy in a vector network analyser, and phase noise or adjacent-channel leakage-power ratio in a signal generator. Next, identify the associated accuracy levels and tolerances for each key specification. Whatever level of accuracy you need, and however tight your tolerances may be, the instruments used for calibration must be even better.

Putting the process into action

If you haven’t done so recently, it might be time to check your roster of key instruments and re-examine the scope of your critical measurements. After that, it will be worthwhile to compare those needs with your current calibration provider’s capabilities and calibration reports.

The key test: Are you getting what you need to ensure your organisation’s ongoing success? As mentioned at the beginning of this article, it’s all about measurements – and the pass/fail decisions – that you expect your test equipment to deliver.

Contact Lizzy Mabaso, Concilium Technologies, Tel 012 678-9202, lizzy@concilium.co.za

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